1. Field of the Invention
The present invention relates generally to the field of communications technology, and specifically to the use of pulses of electromagnetic radiation (EMR) within or in conjunction with fiber-optic communication systems and networks for the transmission of data.
2. Description of Related Technology
Light Propagation Speed Experiments
It has recently been demonstrated that the propagation speed of light pulses can be dramatically affected and controlled, and even effectively “stopped” for short periods of time. See Chien Liu, et al, “Observation of coherent optical information storage in an atomic medium using halted light pulses”, Nature, (January 2001) (advanced publication), herein after referred to as “Liu”; D. F. Phillips, A. Fleischhauer, A. Mair, R. L. Walsworth and M. D. Lukin from the Harvard-Smithsonian Center for Astrophysics, publishing in Physical Review Letters 86, 783 (29 Jan. 2001), hereinafter “Phillips”; Hau, L. V., Harris, S. E., Dutton, Z. & Behroozi, C. H. “Light speed reduction to 17 meters per second in an ultracold atomic gas”, Nature 397, 594±598 (1999); Kash, M. M. et al. Ultraslow group velocity and enhanced nonlinear optical effects in a coherently driven hot atomic gas. Phys. Rev. Lett. 82, 5229±5232 (1999); Budker, D., Kimball, D. F., Rochester, S. M. & Yashchuk, V. V. Nonlinear magneto-optics and reduced group velocity of light in atomic vapor with slow ground
Electromagnetically induced transparency (EIT) has been observed in various atom-gas systems, as is disclosed in, for example, H. R. Gray et al., Opt. Lett. 3, 218 (1978); M. Kaivola et al., Opt. Commun., 49, 418 (1984); A. Aspect et al., Phys. Reve. Lett. 61, 826 (1988); S. Adachi et al., Opt. Commun., 81, 364 (1991); A. M. Akulsin et al., Opt. Commun., 84, 139 (1991); Y. Q. Li et al., Phys. Rev., A51, R1754 (1995); A. Kasapi et al., Phys. Rev. Lett. 74, 2447 (1995). Liu showed that coherent optical information can be stored in an atomic medium and subsequently read out by using the effect of EIT in a magnetically trapped, heavily cooled Bose-Einstein condensed (BEC) sodium atom cloud. The apparatus of Liu al is reproduced (simplified diagram) herein as FIG. 1. It has been experimentally verified by Liu that the repeated and reliable storage of quantum state information associated with a light pulse, and the subsequent “read-out” thereof, are controlled substantially by stimulated photon transfer between two laser fields, specifically those associated with the “probe” resonant pulse and the coupling or interference-producing laser. It has further been experimentally demonstrated that multiple such “read-outs” of a stored pulse can be achieved through the application of a series of short, coupling laser pulses (see, e.g., FIGS. 4a and 4b of Liu cited above). As illustrated in FIGS. 4a and 4b of Liu, measurements of multiple (e.g., double and triple) pulse read-outs spaced by up to hundreds of microseconds may be produced using the aforementioned techniques. Advantageously, each of the regenerated probe pulses in such multiple readouts contains a portion of the contents of the atomic memory, notably in the form of energy (i.e., the total energy of the multiple pulses is equivalent to that for a single read-out pulse obtained using a longer coupling laser pulse). Successive depletion of the “quantum memory” occurs for each successive pulse. As pointed out by Liu, et al., such capability is potentially useful for quantum information transfer. Through injection of multiple such “probe” pulses into a Bose-Einstein condensate (e.g., cooled sodium cloud), in which most atomic collisions are coherence-preserving, quantum information processing may be possible during the storage time.
While at a high level similar to Liu, the approach of Phillips used a rubidium vapor cloud ˜70-90° C. The rubidium vapor was contained in a cell about 4 cm long. Photons from the signal pulse slowed to about 1 km·s−1 in the Phillips vapor cell. “Trapping” of photons for hundreds of microseconds was exhibited. Phillips identifies that the information from the photons is stored or reflected in the spin states of atomic electrons.
Unfortunately, while very compelling, the aforementioned disclosure by Liu (as well as the other references cited) make little if any practical application of their findings.
Optical Communication Systems
Traditional fiber optic communication systems utilize pulses or waves of light energy which propagate in one or more modalities along a conduction medium, typically an optical fiber adapted to carry such pulses or waves efficiently at the desired wavelengths. In long fiber transmission lines, the pulses or waves must be periodically regenerated or amplified due to losses inherent in the transmission medium and the pulses/waves themselves. The amplification function is often performed by amplifiers such as Erbium Doped Fiber Amplifiers (EDFAs) of the type well known in the art. The amplifier is able to compensate for power loss due to signal absorption in the optical fiber, but it is generally unable to correct the signal distortion caused by related to any number of attendant factors including “chirping”, linear dispersion, wave mixing, polarization distortion and other propagation-related or timing effects. Additionally, noise accumulated along the transmission line is not addressed by such EDFAs or other amplifiers.
Wave division multiplexed (WDM) networks typically use modulated lasers as the source of laser light, as previously described. However, the pulses produced by such lasers in the WDM context are often characterized by chromatic dispersion of the pulses. Specifically, the leading and trailing edges of the pulse(s) typically includes multiple frequency components that are changing from one state to another (e.g., “low” to “high”). When transmitted over an optical fiber, such pulses experience a phenomenon commonly referred to as “chirping”, which ultimately reduces the distance a train of closely spaced pulses can be transmitted without overlap between individual pulses in the train. Obviously, such overlap detracts from the signal quality, and the ability to recover the signal information at the receiver end of the transmission line.
One approach to improving the quality of pulses from a directly modulated laser comprises passing the pulses through a narrowband filter to remove unwanted transitional frequency components at the leading and trailing edges of the pulses.
Another approach used in soliton systems comprises a non-return-to-zero (NRZ) electro-absorption optical modulator with a soliton pulse shaper, which attenuates the regions of highest transient chromatic dispersion generated by the NRZ modulator.
In spite of the foregoing filtering/shaping techniques, the optical signal must periodically be regenerated, especially after a cascade of multiple amplifiers (such as would be encountered over a long distance transmission path). Many factors including those relating to the initial laser source, input signal, transmission fiber, and amplifiers contribute to the determination of the distance at which regeneration must occur. Typically, regeneration of the signal is performed with electronic repeaters operating on the principle of optical-to-electronic conversion. However, there are significant drawbacks associated with such conversion, including increased cost, and generally complex and often error-prone supporting/compensating electronics. Ideally, the repeating process would remain entirely in the optical domain.